Certain books offer intelligence (I forget which ones), watching the news I think and things like socialising in museums, etc. I find the most efficient way is to keep intelligent friends and socialise with them frequently - as just a couple. If you socialise with one person as opposed to a group, you can gain as much as ten intelligence points a time.

On the other hand I think intelligence can be lost through drinking and other activities? (I'm sure Cliffski will be able to correct me if i'm wrong, it's been a while since I played).

Yes, just returning to the game after leaving it for quite a while. Books no longer seem to have any effect. I haven't tried the news--perhaps because in reallife the mainstream news media is about as dumbed-down as possible on this side of the Atlantic. ;) But socializing in museums, and finding the right kind of friends, sounds about right. Thanks.